1. Field of the Invention
The present invention relates to a method for manufacturing a lithium secondary battery having a prismatic shape, and more particularly, to a method for making a prismatic lithium secondary battery which has an electrode assembly having a separating film between a positive electrode plate and a negative electrode plate.
2. Description of the Related Art
In forming an electrode assembly for a prismatic lithium secondary battery, conventionally, a positive electrode plate and a negative electrode plate are alternately interposed between separation films and stacked. Then, the stacked plates are passed through between heated rollers and laminated. The reason for laminating the stacked plates is for incorporating the respective plates without being separated from each other.
However, the laminating step necessitates infiltration of an electrolytic solution into the tightly fixed electrode assembly in a subsequent processing step. To solve this problem, conventionally, a plasticizer in a relatively high composition ratio was added to a solution applied to the surfaces of positive and negative electrode current collectors, in the process of forming a positive electrode plate and a negative electrode plate. The composition ratio is generally about 20% by weight. The reason why the plasticizer is added to the solution is to increase the adhesion of the respective plates and to infiltrate the electrolytic solution into a space produced by extracting the plasticizer after forming the electrode assembly.
To sum up, according to the conventional method in which a laminating step is required, the following problems occur. First, since interface resistance between the respective plates increases due to lamination, unnecessary energy consumption is caused due to the interface resistance. Second, since a plasticizer in a relatively high composition ratio is added to a solution used for coating positive and negative electrode plates, the composition ratio of active materials becomes relatively low. Thus, the output capacity of a prismatic lithium secondary battery is reduced and the life thereof is shortened. Finally, since a laminating step and a plasticizer extracting step are necessary, the productivity of prismatic lithium secondary batteries is lowered.
To solve the above problems, it is an object of the present invention to provide a method for manufacturing a lithium secondary battery of a prismatic type, by which the internal energy consumption can be reduced, and the output capacity and cycle life of the battery can be improved, and the productivity thereof can be enhanced.
Accordingly, to achieve the above object, there is provided a method for manufacturing a lithium secondary battery of a prismatic type having an electrode assembly having a separation film between a positive electrode plate and a negative electrode plate, the method including the steps of forming a plurality of separation films, and positive and negative plates having a narrower width than the separation films, alternately interposing the positive and negative plates between the separation films and stacking the same, hermetically sealing both sides of the separation films by heating along the lengthwise direction to form an electrode assembly, and immersing the electrode assembly in an electrolytic solution and putting the same into a case to then be hermetically sealed.
According to the present invention, since the both sides of the separation films are hermetically sealed by heating along the lengthwise direction, the respective plates can be incorporated without being separated from each other for performing subsequent steps. Thus, unlike in the conventional technology, no lamination is necessary in the present invention.